How to get integer productId of an unsubmitted In-App-Purchase?

In order to use SKStoreProductViewController buy(int productid) method provided by Xamarin tutorial page, I want to get the product Id of the In-App-Purchase item.

Now the state is that the In-App-Purchase item is created on iTunes Connect but not submitted, because I haven't test it in sandbox.

Hope someone can help. Thank you very much.